1. Field of the Invention
The present invention relates to piezoelectric electroacoustic transducers such as a piezoelectric receiver and a piezoelectric sounder.
2. Description of the Related Art
Piezoelectric electroacoustic transducers, for use as either a piezoelectric sounder or a piezoelectric receiver for generating a warning or an operating sound, have been widely used in electronic apparatuses, electric appliances, mobile phones, and other devices. Such a piezoelectric electroacoustic transducer is generally constructed such that a unimorph piezoelectric diaphragm is formed by bonding a round piezoelectric element to one side of a round metal plate. The periphery of the rear surface of the metal plate is supported by a silicone rubber in a round case, and the opening of the case is closed by a cover.
However, the round diaphragm has problems in that its production efficiency and electroacoustic transducing efficiency are both low and also it is difficult to reduce its size.
In view of the problems described above, a surface-mounted piezoelectric electroacoustic transducer using a square diaphragm in place of the round diaphragm is disclosed in Japanese Patent Application Publication No. 2000-310990 so as to improve both production efficiency and electroacoustic transducing efficiency and to reduce its size. The piezoelectric electroacoustic transducer has the square piezoelectric diaphragm, an insulating case, supports for supporting the diaphragm, and a cover plate having a sound hole therein. The supports are disposed on two mutually opposing walls of the case and have external connection terminals. The diaphragm is housed in the insulating case. Two mutually opposing sides of the diaphragm are fixed to the supports with an adhesive or an elastic sealant, and the gaps between the other two sides of the diaphragm and the case are sealed with the elastic sealant. Also, the diaphragm and the terminals are electrically connected to each other with a conductive adhesive, and the cover plate is bonded to the upper surfaces of the sidewalls of the case.
Although the above-described electroacoustic transducer uses a unimorph piezoelectric diaphragm, a piezoelectric electroacoustic transducer using a piezoelectric diaphragm made of laminated piezoelectric ceramics is also disclosed in Japanese Patent Application Publication No. 2001-95094.
In the known piezoelectric electroacoustic transducers, two sides of the diaphragm are fixed to the case. In addition, the gaps between the case and the other two sides or all four sides of the diaphragm are sealed with an elastic sealant in order to separate the front and rear sides of the diaphragm so that two independent acoustic spaces are formed in the front and rear of the diaphragm. A soft elastic material such as a silicone rubber is used as the elastic sealant so as to minimize the vibration of the diaphragm.
FIGS. 12 and 13 are sectional views of sealing portions, using high-viscosity and low-viscosity elastic sealants, respectively, of the known piezoelectric electroacoustic transducers.
Any of the elastic sealants is filled in the gap between the side surface of the diaphragm and the inner surface of the case and is then cured. When a room-temperature-setting silicone rubber is used as an elastic sealant 42, as shown in FIG. 12, the gap between a diaphragm 40 and a case 41 is easily sealed since the elastic sealant 42 is quickly cured after being filled. However, the room-temperature-setting elastic sealant 42 causes an applicator to become easily clogged since the sealant 42 begins to be cured during its filling process, thereby leading to a poor workability. In addition, the sealant 42 has a high Young""s modulus after being cured, thereby lessening the vibration of the diaphragm 40.
On the contrary, when a thermosetting silicone rubber having a low viscosity (i.e., a low thixotropy) is used as an elastic sealant 43, the elastic sealant 43 does not begin to be cured during its filling process and has a low Young""s modulus, thereby providing an advantage in that the vibration of the diaphragm 40 is not reduced.
However, as shown in FIG. 13, the low-viscosity elastic sealant 43 flows to the bottom of the case 41, thereby causing a problem in that the gap between the diaphragm 40 and the case 41 is not sealed.
In order to overcome the problems described above, preferred embodiments of the present invention provide a piezoelectric electroacoustic transducer which reliably seals the gap between a diaphragm and a case and which provides excellent vibration characteristics of the diaphragm, even when the piezoelectric electroacoustic transducer uses a low-viscosity elastic sealant.
A piezoelectric electroacoustic transducer according to a preferred embodiment of the present invention includes two electrodes, a substantially square piezoelectric diaphragm producing a bending vibration in the thickness direction thereof in response to application of an alternating voltage between the two electrodes, a housing for housing the piezoelectric diaphragm, an elastic sealant for sealing the gap between the periphery of the diaphragm and the inner side surface of the housing, supports, disposed in the housing, for supporting at least two mutually opposing sides of the piezoelectric diaphragm or the four corners of the piezoelectric diaphragm, grooves which are disposed in the housing so as to face the periphery of the rear surface of the piezoelectric diaphragm and in which the elastic sealant is filled, and walls defining inner portions of the grooves, disposed such that the upper surfaces of the walls are located below those of the supports, for preventing the elastic sealant from flowing out to the bottom of the housing.
When a low-viscosity elastic sealant is used for filling the gap between the periphery of the diaphragm and the inner side surface of the housing, the elastic sealant tends to flow out through the gap between the diaphragm and the housing. However, in the piezoelectric electroacoustic transducer according to a preferred embodiment of the present invention, the elastic sealant flows into the grooves disposed in the housing, and, even when the elastic sealant is poured over the grooves, the walls defining inner portions of the grooves, for preventing the elastic sealant from flowing out, blocks the overflown elastic sealant so as to prevent the sealant from flowing out. Accordingly, the elastic sealant is prevented from flowing out to the bottom of the housing and thus remains in the gap between the periphery of the diaphragm and the inner side surface of the housing, thereby reliably sealing the gap therebetween.
Since the upper surfaces of the walls are located below those of the supports for supporting the diaphragm, the walls do not come to contact with the rear surface of the diaphragm and thus do not lessen or hinder the vibration of the diaphragm, consequently leading a piezoelectric electroacoustic transducer having excellent vibration characteristics.
In the piezoelectric electroacoustic transducer according to preferred embodiments of the present invention, gaps are preferably formed between the upper surfaces of the walls and the rear surface of the diaphragm so as to have a dimension such that the elastic sealant does not flow out through the gaps because of its surface tension. For example, when the viscosity of the uncured elastic sealant is about 1300 mPaxc2x7s, the foregoing dimension is preferably about 0.2 mm or less, since a dimension that is greater than the above figure may cause the elastic sealant to flow out to the bottom of the housing.
The piezoelectric electroacoustic transducer according to preferred embodiments of the present invention may further include a case having a substantially U-shape cross-section, a bottom, and side walls, a cover plate, and tapered projections, disposed on the inner surfaces of the side walls of the case, for guiding the periphery of the piezoelectric diaphragm, wherein the housing is defined by the cover plate being bonded to the upper surfaces of the sidewalls of the case.
The piezoelectric diaphragm produces a bending vibration in the thickness direction thereof in response to application of an alternating voltage between the two electrodes. When the periphery of the diaphragm comes into contact with the inner side surfaces of the side walls of the case at large areas, the vibration of the diaphragm is decreased and, as a result, its acoustic pressure decreases.
As a solution to this problem, the tapered protrusions which come into contact with the periphery of the diaphragm at small areas are disposed on the inner side surfaces of the sidewalls of the case so as to prevent the vibration of the diaphragm from being decreased or hindered. Also, since the protrusions have a function of guiding the diaphragm, the differences between the inner dimensions of the case and the outer dimensions of the diaphragm can be minimized, thereby achieving a small piezoelectric electroacoustic transducer.
The piezoelectric electroacoustic transducer according to preferred embodiments of the present invention may further include a case having a substantially U-shaped cross-section, a bottom, and side walls, a cover plate, and recesses, disposed along the upper inner edges of the sidewalls of the case, for preventing the elastic sealant from flowing upwardly, wherein the housing is defined by the cover plate being bonded to the upper surfaces of the sidewalls of the case.
When the elastic sealant flows up to the upper surfaces of the sidewalls while the cover plate is bonded to the upper surfaces of the sidewalls, the bonding strength of the cover plate decreases, as a result, sometimes causes air-leakage of an acoustic space formed in the upper portion of the diaphragm. As a solution to this problem, the recesses are formed along the upper inner edges of the sidewalls so as to prevent the elastic sealant from flowing upwardly, thereby allowing the cover plate to maintain its bonding strength.
As is clear from the above description, in the housing of the piezoelectric electroacoustic transducer according to preferred embodiments of the present invention, the grooves in which the elastic sealant is filled are formed and the walls for preventing the elastic sealant from flowing out are also formed as inner portions of the grooves. This configuration prevents the elastic sealant from flowing out to the bottom of the housing even when the elastic sealant has a low viscosity, and thus the elastic sealant reliable seals the gap between the periphery of the diaphragm and the inner side surface of the housing. As a result, both the manufacturing workability and the sealing performance of the piezoelectric electroacoustic transducer can be improved, and also the vibration characteristics of the same can be improved since the elastic sealant has a reduced Young""s modulus after being cured.
Also, since the upper surfaces of the walls for preventing the elastic sealant are located below those of the supports for supporting the diaphragm, the walls do not come into contact with the rear surface of the diaphragm, thereby preventing the vibration of the diaphragm from being decreased or hindered.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings.